In EUV lithography, highly integrated structures with a line width of less than 50 nm are produced by microlithographic projection devices. Laser radiation from the EUV range (extreme ultraviolet light, also called soft X-ray radiation) is used at wavelengths around 13 nm. The projection devices are equipped with mirror elements which consist of glass having a high silicic acid content and doped with titanium oxide (hereinafter also called TiO2—SiO2 glass) and which are provided with a reflective layer system. These materials are distinguished by an extremely low linear thermal expansion coefficient (shortly called “CTE”; coefficient of thermal expansion) which is adjustable through the concentration of titanium. Standard titanium oxide concentrations are between 6% by wt. and 9% by wt.
Such a blank of synthetic, titanium-doped glass with a high silicic acid content and a manufacturing method therefor are known from DE 10 2004 015 766 A1. The TiO2—SiO2 glass is produced by flame hydrolysis of titanium- and silicon-containing start substances and contains 6.8% by wt. of titanium oxide. It should be noted that the hydroxyl group content of the glass produced in this way is seldom below 300 wt. ppm. To increase the radiation resistance of the glass, it has been suggested that the concentration of the hydrogen present due to the manufacturing process should be lowered by heating to values below 1017 molecules/cm3. To this end, the glass is heated to a temperature in the range between 400° C. and 800° C. and kept at this temperature for up to 60 hours. One of the plane surfaces of the mirror substrate is provided with a reflective coating, with a plurality of layers being produced one on top of the other.
In the intended use of the mirror substrate, the upper side thereof is provided with a reflective coating. The maximum (theoretical) reflectivity of such an EUV mirror element is about 70%, so that at least 30% of the radiation energy is absorbed in the coating or in the near-surface layer of the mirror substrate and converted into heat. This leads in the volume of the mirror substrate to an inhomogeneous temperature distribution with temperature differences that, according to data given in the literature, may amount to up to 50° C.
Therefore, it would be desirable for a deformation that is as small as possible that the glass of the mirror substrate blank has a CTE that is zero over the whole temperature range of the working temperatures occurring during use. In the case of Ti-doped silica glasses, the temperature range with a CTE around zero can, however, be actually very narrow.
The temperature at which the coefficient of thermal expansion of the glass is equal to zero shall also be called zero crossing temperature or TZC (temperature zero crossing) hereinafter. The titanium concentration is normally set such that one obtains a CTE of zero in the temperature range between 20° C. and 45° C. Volume regions of the mirror substrate with a higher or a lower temperature than the preset TZC expand or contract, resulting, despite an altogether low CTE of the TiO2—SiO2 glass, in deformations that are detrimental to the imaging quality of the mirror.
Suggestions have therefore been made for counteracting the deterioration of the optical imaging caused by inhomogeneous temperature distribution in the mirror substrate blank. A metallic substrate material is for instance provided in the mirror known from EP 0 955 565 A2. Thanks to the high thermal conductivity of the metal, the heat introduced into the mirror is efficiently discharged via the back side of the metal substrate, preferably by a cooling device.
Although it is thereby possible to reduce thermally induced mirror deformations, image errors cannot be avoided. Substantial aberrations are still found.
DE 103 59 102 A1 (US 2005/0185307 A1) defines homogeneity requirements for a SiO2—TiO2 glass that are to be satisfied by the glass. For this purpose, the glass shall have a location-dependent thermal longitudinal expansion coefficient defined by the titanium content. Moreover, this coefficient shall be independent as much as possible of the temperature, defined by the amount of the mean rise m of less than 1.5×10−9 K−2. However, it is not indicated how this low temperature dependence of the CTE is achievable.
According to WO 2011/078414 A2, in a blank for a mirror substrate or for a mask plate of SiO2—TiO2 glass, the concentration of titanium oxide over the thickness of the blank is to be adapted step by step or continuously to the temperature distribution arising during operation in such a manner that the condition for the zero crossing temperature TZC is satisfied at every place, i.e. the coefficient of thermal expansion for the locally evolving temperature is substantially equal to zero. A CTE is here defined as being substantially equal to zero if the remaining longitudinal expansion is 0+/−50 ppb/° C. at every place during operation. This is to be accomplished in that during production of the glass by flame hydrolysis, the concentration of titanium- and silicon-containing start substances is varied such that a predetermined concentration profile is obtained in the blank.
The methods for optimizing the TZC by local variation of the titanium concentration require precise knowledge of the temperature distribution arising during use over the volume of the component to be optimized and entail enormous design and adaption efforts for the individual component. It should here be noted that a projection lens system contains a plurality of mirrors of different sizes and shapes that have not only flat, but also convexly or concavely curved surfaces which are provided with a reflective coating and have outer contours adapted to the specific use. The temperature profile over the volume of each component to be optimized that is really achieved during operation depends on the specific conditions of use and on the environment and can be determined exactly only in the fully mounted projection lens system under real conditions of use. Technically, however, it is hardly possible to exchange individual components of a fully mounted projection lens system.
This is aggravated by the fact that the CTE and thus the scalable TZC depend, apart from the titanium oxide content, also on the hydroxyl group content and on the fictive temperature of the glass. The fictive temperature is a glass property that represents the degree of order of the “frozen” glass network. A higher fictive temperature of the TiO2—SiO2 glass is accompanied by a lower degree of order of the glass structure and a greater deviation from the energetically most advantageous structural arrangement.
The fictive temperature is influenced by the thermal history of the glass, particularly by the latest cooling process. There are bound to be other conditions for near-surface regions of a glass block than for central regions, so that different volume regions of the mirror substrate blank already have different fictive temperatures due to their different thermal histories. The distribution of the fictive temperature over the blank volume is therefore always inhomogeneous. A certain equalization of the profile of the fictive temperature is achievable by way of annealing. However, annealing processes are troublesome in terms of energy and time.
This is further aggravated by the fact that the resulting fictive temperature also depends on the composition of the TiO2—SiO2 glass, and particularly on the hydroxyl group content and the titanium oxide concentration. Even with a very careful and long annealing process, the profile of the fictive temperature over the blank volume cannot be homogenized when the composition is not completely homogeneous. This, however, is not readily applicable especially in the case of a hydroxyl group content that can be varied by drying measures.